1. Field of the Invention
The present invention relates to an optical isolator that utilizes nonreciprocity and, more particularly, to a polarization independent optical isolator particularly suitable for optical communications.
2. Description of the Related Art
An optical isolator has directivity in its function so as to allow forward propagation of light but blocks backward propagation of light. A laser diode (hereinafter abbreviated as LD) used in optical communications or optical measurements, when reflected light returning from the outside enters an active layer of the LD, experiences the collapse of internal interference leading to such troubles as wavelength deviation and fluctuation in the output power. In order to prevent the reflected light from returning so that the LD maintains stable oscillation, an optical isolator that prevents light from propagating backward is used. The optical isolator is indispensable in such applications as high-precision measurements, communications with high-speed modulation and high-density communications that require strict control of the wavelength.
FIGS. 2A and 2B show the constitution and operation of a polarization independent optical isolator. The optical isolator comprises an input optical fiber 13, a first birefringent crystal 2, a Faraday rotator 5, a second birefringent crystal 3, a third birefringent crystal 4 and an output optical fiber 14. Although there are lenses provided between the input optical fiber 13 and the first birefringent crystal 2, and between the third birefringent crystal 4 and the output optical fiber 14 in an actual setup, they are omitted here for simplicity.
Drawings (A) through (E) of FIG. 2A and FIG. 2B show the positions of light rays and the directions of polarization at positions of (A) through (E). Drawings (A) through (E) of FIG. 2A and FIG. 2B show the states viewed from the left-hand side of the drawing, where the center of an arrow indicates the position of the light ray and the orientation of the arrow indicates the direction of polarization.
FIG. 2A shows the operation for forward propagation. Light ray emerging from the input optical fiber 13 is in a state of mixed polarization at position (A). This light ray is split into ordinary light ray and extraordinary light ray when passing through the first birefringent crystal 2, with the extraordinary light ray being shifted by birefringence. Consequently, there are two light rays that have been split at position (B). Then the direction of polarization is rotated by 45 degrees when passing through the Faraday rotator 5, thus assuming the state shown in (C). Direction of polarization is further rotated by 45 degrees when passing through the second birefringent crystal 3, thus assuming the state shown in (D). Then, when passing through the third birefringent crystal 4, the light rays are synthesized by birefringence, thus assuming the state shown in (E). The synthesized light ray propagates toward the output optical fiber 14.
FIG. 2B shows the operation for backward propagation. Light ray emerging from the output optical fiber 14 is in a state of mixed polarization at position (E). This light ray is split by birefringence into two light rays when passing through the third birefringent crystal 4 as shown in (D). Then the polarization plane is rotated by 45 degrees when passing through the second birefringent crystal 3. Since this rotation is a reciprocal rotation that is dependent on the direction of light propagation, the polarization plane is rotated in a direction opposite to that of the case of forward propagation shown in FIG. 2A, thus assuming the state shown in (C). Then the polarization plane is rotated by 45 degrees when passing through the Faraday rotator 5. Since this rotation is a nonreciprocal rotation that is independent of the direction of light propagation, the polarization plane is rotated in the same direction as in the case of forward propagation shown in FIG. 2A, thus assuming the state shown in (B). The state of (B) is reverse to the state of forward propagation. As a result, separation of the ordinary light ray and the extraordinary light ray becomes greater when passing through the first birefringent crystal 2, thus assuming the state shown in (A). As a consequence, there is generated a deviation in position from the input optical fiber 13, thus resulting in a reverse propagation loss.
The polarization independent optical isolator that uses the birefringent crystal functions through the use of shifting light ray as described above, and therefore the input optical fiber 13 and the output optical fiber 14 are staggered with respect to each other in the constitution described above.
Japanese Unexamined Patent Publication (Kokai) No. 8-194130 discloses an example where a compact optical isolator is constituted by using a thermal expansion core fiber instead of lens. In this example, an optical isolator element is tilted so as to correct the shifting of beam to prevent misalignment from occurring between the thermal expansion core fibers on the input and output sides.
Japanese Unexamined Patent Publication (Kokai) No. 9-54283 discloses an optical isolator that similarly uses a thermal expansion core fiber. In this example, too, in order to prevent misalignment from occurring between the thermal expansion core fibers on the input and output sides, wedge-shaped polarizing beam splitter is used in addition to tilting the optical isolator element.
Japanese Unexamined Patent Publication (Kokai) No. 8-286150 discloses an example where a rod lens is used instead of the thermal expansion core fiber. This document describes a constitution where the end face of the rod lens is tilted in order to prevent back reflection.